Russian forces are increasingly relying on heavy Soviet-era weapons that can cause mass casualties and significant collateral damage in their bid to make headway in capturing eastern Ukraine.
Fierce, prolonged fighting is depleting resources on both sides, according to British and US defence officials.
Russian bombers are thought to have been launching hefty 1960s-era anti-ship missiles in Ukraine, the UK Defence Ministry said on Saturday.
The Kh-22 missiles, with a reported range of 1,000 kilometres, were primarily designed to destroy aircraft carriers and can carry a nuclear warhead.
When used in ground attacks with conventional warheads, they "are highly inaccurate and therefore can cause severe collateral damage and casualties," the ministry said.
It is unclear how many Kh-22s Russia has in its arsenal. By 2006, Ukraine has scrapped 423 missiles it inherited from the Soviet Union after deciding to decommission the weapon.
Both sides have expended large amounts of weaponry in what has become a grinding war of attrition for the eastern region of coal mines and factories known as the Donbas, placing huge strains on their resources and stockpiles.
Russia is likely using the 5.5-tonne anti-ship missiles because it is running short of more precise modern missiles, the UK ministry said. It gave no details of where exactly such missiles are thought to have been deployed.
As Russia also sought to consolidate its hold over territory seized so far in the 108-day war, the US defence secretary said Moscow's invasion of Ukraine "is what happens when oppressors trample the rules that protect us all".
"It's what happens when big powers decide that their imperial appetites matter more than the rights of their peaceful neighbors," Lloyd
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